This invention relates generally to an improved diaphragm of an axial flow turbine using an elastic fluid, such as steam, and more particularly to control of supersonic transitions and flow of elastic fluid through the diaphragm.
A diaphragm of an axial flow turbine typically comprises an inner and an outer circumferential ring and a plurality of spaced apart nozzle partitions for forming elastic fluid flow passages therebetween. Each nozzle partition includes an end respectively fixedly secured to the inner and outer ring, respectively, of the diaphragm. In operation, the outer ring is generally fixedly mounted to the inner shell of the turbine and the inner ring is spaced from and surrounds the rotor of the turbine. Typically, some type of seal, such as labyrinth seals known in the art, are disposed between the inner ring of the diaphragm and the rotor of the turbine. Nozzle partitions control and direct flow of elastic fluid into energy extracting means, such as turbine blades or buckets, and a cooperating combination including a diaphragm and a plurality of buckets is commonly referred to as a stage.
In order to obtain maximum power from energy available from the elastic fluid, it is necessary that the flow of elastic fluid be precisely controlled. The flow of elastic fluid must impinge the energy extracting means at a predetermined optimum angle and the optimum elastic fluid flow distribution from the radial inner portion or root of the nozzle partition to the radial outer portion or tip of the nozzle partition must be maintained and efficiently accommodate a broad range of operating conditions, such as elastic fluid mass flow rate variations and stage output pressure variations, which may be expected, especially for the last stage of a low pressure turbine.
It is possible to obtain supersonic steam flow through passages between nozzle partitions of a diaphragm in a steam turbine, especially at the root (radially inner portion) of the last stage of a low pressure turbine, and the transition or transonic region from subsonic to supersonic flow must be controlled to ensure that desired steam flow conditions, such as minimizing oblique shocks to minimize efficiency loss resulting therefrom, are maintained throughout the stage from the input of the nozzle partitions to the input of the buckets and ultimately to the input of the next stage or a condenser if the steam output from the buckets is from the last stage. An improper or unexpected transonic region through passages between nozzle partitions may result in a loss of efficiency due to undesirable shock patterns. A transition from subsonic to supersonic flow is accompanied by a shock wave which causes an irreversible loss of pressure, i.e., pressure is lost and cannot be recovered to produce mechanical energy. It is especially worthwhile to ensure that operation of the last stage of a low-pressure steam turbine yields optimum stage (and thereby optimum diaphragm) efficiency since the last stage of a low pressure turbine recovers substantially more energy, typically about 10% of the overall turbine output, from steam than any other stage in the turbine and thus has a significant impact on overall efficiency of the turbine.
Natural forces, such as those due to rotation of turbine components, tend to direct steam flow radially outward, away from the inner portion or root of buckets, thereby creating flow separation and potential starvation at the root of buckets. It would be desirable to redirect at least some steam flow radially inward in order to delay onset of flow separation and starvation.
It is an object of the present invention to provide a diaphragm for an axial flow turbine for controlling transonic flow of elastic fluid through the diaphragm.
Another object is to provide a diaphragm for maintaining desired radial flow distribution of elastic fluid through the diaphragm and at the output of the diaphragm over a range of operating conditions.
Yet another object is to provide a diaphragm for directing a proportionally greater amount of elastic fluid flow radially inwardly to minimize bucket root starvation and to delay onset of flow separation and recirculation.